MECHANICAL PROPERTIES OF CONTINUOUSLY ANNEALLED Si–FREE P–CONTAINING TRIP STEEL

Abstract

Since its advent, TRIP steel has been considered as the best automotive steel to improve the automotive safety dramatically because of its excellent mechanical properties. However, TRIP steel was not widely used and the main reason is that the large amount of Si in the steel led to surface quality problems of the plate. In this work a kind of C-Mn-Al-P TRIP steel with Si being replaced by Al and P was designed and subjected to continuous annealing. The macro and micro mechanical performance was analyzed. The results showed that the mechanical properties were excellent with tensile strength between 740 and 790 MPa and elongation above 25%, indicating that the production of TRIP steel without Si addition was feasible. Compared with traditional TRIP steel with Si, the designed steel demonstrated higher yield strength of 550—600 MPa, which was a little higher than that of high Si steel and much higher than that of medium or low Si steel. The micro hardness of ferrite was measured by nano mechanics probe and it was 0.6 GPa higher than that of medium Si TRIP steel. The IF steel whose strength was already known was used as a calibration and the conclusion was that the 0.6 GPa difference in nano hardness could result in 75 MPa difference in yield strength and 80 MPa division in tensile strength. Through further analysis, it was believed that the higher strength of ferrite was mainly caucsed by C, not P.